This slide rule was produced for
the Steel Castings Research & Trade Association (SCRATA) by Blundell Harling
Limited of Weymouth England. It's purpose was to calculate the
appropriate exposure times in gamma radiography.

The factors that the slide rule took into account were the source nuclide
(cobalt, thulium, cesium or iridium), the source activity, the source to
film distance, as well as the thickness and density of the material being
"x-rayed." It even calculated the decay of iridium
sources!
The earlier (1950s) version was designed to be used with radon sources as
well as the four sources mentioned previously.